The principle of recording information on a magnetic recod carrier is well known. It consists in using at least one magnetic recording head to magnetise the record carrier, which is moving close to the said head.
It will be recalled that in order to magnetise a magnetic material, it is first subjected to a magnetic field the strength of which is sufficiently great to saturate the material, that is to say sufficiently great that the induced magnetism in the material reaches a limiting value Bs as soon as the strength H of the magnetic field reaches a given value Hs. The magnetic field is then cut off. Within the material there then remains a certain level of induced magnetism, termed the residual flux density.
Magnetic record carriers are of two types, namely:
Longitudinally magnetised carriers in which the orientation of the induced magnetism is parallel to the surface of the carrier; and PA1 Transversely magnetised carriers in which the orientation of the induced magnetism is perpendicular to the surface of the carrier. PA1 The benefit of the maximum magnetic field in the pole gap is lost; PA1 Since the distance between the magnetic head and the record carrier is small, there is a possibility of contact between the carrier and the head, and hence a danger of the carrier wearing quickly, which makes it necessary for the head to be machined and fitted to very close tolerances; PA1 The electromagnetic efficiency of the head is very low, since when a large amount of power is applied to the coil, only a small magnetic power is received; PA1 The high cost of the head.
In present day practice, arrangements for recording information on magnetic record carriers are formed by the combination of a longitudinally magnetised magnetic record carrier and at least one recording head formed by an electromagnet.
The pole gap is a very narrow opening the width of which does not exceed a few microns so that the lines of induced force are concentrated in the air.
There are, however, lines of force which close the magnetic circuit of the electrogmagnet between the two poles outside the pole gap and which represent magnetic leakage at the tap. The strength of the leakage decreases with distance from the pole gap. It is through this leakage field that the record carrier moves past the head. When a current is passed through the coil of the electromagnet a magnetic field is set up in the said magnetic circuit.
To magnetise the carrier properly it is necessary to use a relatively high current. To record a series of items of information on the carrier, the coil is fed with a current of varying strength, which creates on the carrier a succession of small magnetic areas termed "elementary magnetic areas" the size of which is of the same order as the pole gap.
Such arrangements for recording information on magnetic record carriers have the following disadvantages:
Such arrangements, although well suited to magnetic record carriers having high information densities (that is to say on which the number of elementary magnetic areas per unit of length is high), are less well suited to carriers having low and medium information densities, where the length of the elementary magnetic areas is very much greater than that of the pole gap.
In this latter case, information can only be recorded by causing the carrier to move continuously at a constant speed. It is therefore impossible to envisage quasi-instantaneous recording which would enable the magnetic carrier to move forward step-by-step and data item by data item.
It is an object of the present invention to enable these disadvantages to be substantially reduced or overcome.